The present invention relates to coating compositions suitable for the production of coatings that are permeable to water vapor. These compositions contain (A) film-forming polymers that may carry reactive groups, (B) hydrophilic polyaddition products dispersed in component (A) selected from polyurethanes, polyureas, and polyurethane ureas, and, optionally, (C) cross-linking agents.
The preparation of polyurethane-based coatings that are permeable to water vapor is known and is carried out mainly by physical manipulations. To produce these coatings, the polyurethane is dissolved in a solvent and applied as a layer to a substrate or a separating support and the resultant layered composition is passed in the wet state through a bath containing a substance that is miscible with the solvent but is a non-solvent for the polyurethane. The non-solvent enters the solution layer, in which the polyurethane gradually precipitates. When the resultant solidified film is dried, the solvent which escapes, as well as the non-solvent, leave behind microchannels that are then available in the coating for the transport of water vapor.
Coatings having microcavities can be similarly produced by the addition of powders of water-soluble salts to polymer solutions. After formation of the coatings, the salt can be washed out with water, leaving behind microcavities.
The perforation of compact polymer films by means of high energy electron radiation produces films with high permeability to water vapor which can be laminated. This method, however, requires very expensive apparatus.
A variation of the microporous coating technique that dispenses with the complicated immersion bath technology but produces substantially the same results has recently been carried out. In this process--also known as "evaporation coagulation"--water is added to the solution of the polymer in a low boiling organic solvent in such quantity that the resultant spreadable paste still has just sufficient stability and can be applied. Upon evaporation, the organic solvent is first eliminated, and the water (which progressively increases proportionately) precipitates the polymer as in the immersion process and finally escapes upon drying to leave a microporous structure in the film.
The immersion bath processes have the same major disadvantage as evaporation coagulation in that microchannels or microcavities weaken the water vapor-permeable coatings. Consequently, the coatings have distinctly lower tensile strength and abrasion resistance than a solid film.
No shortage of attempts have, therefore, been made to produce permeability to water vapor in a coating not only by applying physical methods but also by chemical means. Thus, for example, it has already been proposed to produce such coatings with polyurethanes consisting partly of water-soluble or hydrophilic starting components.
Thus, German Offenlegungsschriften 1,220,384 and 1,226,071 describe polyurethane coating compositions in which polyurethanes have been prepared from glycols, diisocyanates, and polyethylene glycols having molecular weights of about 1000.
Polyethylene glycols may also be used as diol components in polyester polyols for polyurethane elastomers with a view to producing coatings which are permeable to water vapor and have little tendency to swell in water, as disclosed in Japanese Patent Application 61/009,423.
Compact top coats on composite materials of textile substrates and microporous coatings such as described in German Offenlegungsschrift 2,020,153 are also permeable to water vapor.
Segmented polyurethane elastomers of polyethylene glycols are disclosed in European Patent Application 52,915.
Other organic hydrophilic components have also been added to polyurethanes to produce composite materials and coatings which are permeable to water vapor. In particular, poly-.gamma.-methylglutamate may be added to polyurethanes, used either as starting components or applied to the polyurethanes by dripping. The numerous literature references describing this use include German Offenlegungsschriften 1,922,329 and 1,949,060 and Japanese Patent Applications 58/057,420 and 59/036,781.
The substances which have recently acquired particular technical interest in the field of compact coatings that are permeable to water vapor are mainly polyurethanes which contain the above-mentioned polyethylene glycols as starting components. These raw materials are inexpensive, generally available, and technically easily obtainable. The polyurethanes and polyurethane ureas obtained from these materials are also generally well known. In contrast to the widely used polyurethanes and polyurethane ureas that contain polyester diols, polycarbonate diols, or polyether diols as relatively high molecular weight diols, the above-mentioned polyurethanes are water absorbent, permeable to water vapor, and in some cases even highly swellable or soluble in water. Hydrophobic polyols are therefore added to the polyethylene glycols which cause the hydrophilic character. These mixtures can be considered as an attempt to produce polyurethanes and polyurethane ureas that combine good permeability to water vapor with high resistance to the effects of liquid water.
German Offenlegungsschrift 2,902,090 and U.S. Pat. No. 4,248,756 describe two-component coating compositions of ketoxime-blocked prepolymers and dinuclear, cycloaliphatic diamines as cross-linking agents that are prepared with at most 15% by weight of solvent. The polyhydroxyl compounds may, for example, contain polypropylene oxides and, optionally, also ethylene oxide units incorporated in the blocked prepolymers.
Blocked prepolymers described in European Patent Application 100,005 obtained from polyhydroxyl compounds containing from 20 to 100% by weight (preferably from 40 to 80% by weight) of ethylene oxide units give rise to coatings with good permeability to water vapor after they have been applied to textile substrates and cured. Due to swelling of these layers in water, however, pustule-like swellings appear when discrete droplets of water are placed on the layer. These pustules are not only aesthetically displeasing in a textile or leather (or leather-like) article of use but are also a serious technical defect.
It was surprisingly found that coatings that are permeable to water vapor but have very little tendency to swell in water can be obtained by using coating compositions consisting of polymers that are normally hydrophobic but contain dispersed hydrophilic organic fillers. These coating compositions may be obtained by preparing a hydrophilic organic filler in situ in a blocked NCO-prepolymer or in a completed polyurethane or a polymer.